1. Field of the invention
The present invention relates to an electrodeionization apparatus used for producing deionized water in various fields including semiconductor manufacturing, liquid crystal display manufacturing, pharmaceutical manufacturing, food processing, electric power generation, private device, research establishments and the like, particularly to an electrodeionization apparatus. More particularly, the present invention relates to the electrodeionization apparatus which removes weakly-ionized species electrolytes including silica and boron at a high rate, and is suitable to be employed by a primary pure water system and a reclaim system of pure water producing apparatus.
Furthermore, the present invention relates to an apparatus for producing purified water which employs the electrodeionization apparatus of the present invention so that the apparatus provides the product water of high quality having a resistivity of more than 18.0Mxcexa9xc2x7cm.
2. Description of the Related Art
The electrodeionization apparatus used for producing the deionized water is employed in various fields including the semiconductor manufacturing plants, the liquid crystal display manufacturing plants, the pharmaceutical manufacturing industry, the food processing industry, the electric power industry, the private devices, the research establishments and the like.
FIG. 3 shows an electrodeionization apparatus disclosed in JPH4-72567B, JP2751090, and JP2699256 in which a plurality of anion exchange membranes 13 and a plurality of cation exchange membranes 14 are alternately arranged between electrodes (anode 11, cathode 12) in such a manner as to alternately form concentrating compartments 15 and diluting compartments 16, and the diluting compartments 16 are filled with anion exchangers 10 and cation exchangers 10 comprising ion exchange resins, ion exchange fibers or graft exchangers in mixed or multi-layered form. In FIG. 3, the sign 17 denotes an anodic compartment and the sign 18 denotes a cathodic compartment.
In the electrodeionization apparatus, H+ ions and OHxe2x88x92 ions are formed by dissociation of the water to continuously regenerate the ion exchangers filled in the diluting compartments so that the electrodeionization apparatus can efficiently deionize the water.
FIG. 12 is an exploded view showing the structure of the electrodeionization apparatus.
The electrodeionization apparatus includes a cathode end plate 101, a cathode 102 extending along the end plate 101, a cathode spacer 103 extending along the outer periphery of the cathode 102 which are superposed in this order. Further, a cation-exchange membrane 104, a frame 105 for defining a diluting compartment., an anion-exchange membrane 106, and a frame 107 for defining a concentrating compartment are superposed on the cathode spacer 103 in this order. The cation-exchange membrane 104, the frame 105 for defining a diluting compartment, the anion-exchange membrane 106, the frame 107 for defining a concentrating compartment compose one unit. The apparatus is composed of a plurality of such units superposed together. That is, membranes 104, frames 105, membranes 106, and frames 107 are repeatedly superposed one unit over the other unit. An anode 109 is superposed between the last anion-exchange membrane 106 and an anode spacer 108. An anode end plate 110 is superposed on the anodic electrode 109. The apparatus is tightened by bolts or the like.
The space defined by the inner surface of the frame 105 is the diluting compartment in which an ion exchanger 105R such as ion-exchange resin is filled. The space defined by the inner surface of the frame 107 is the concentrating compartment in which a spacer including a mesh spacer is disposed.
A direct electric current is supplied to pass between the anode 109 and the cathode 102, raw water to be treated is fed to the diluting compartment through a raw water inlet line 111, and feed water is fed to the concentrating compartment through a concentrate inlet line 112. The raw water fed to the diluting compartment flows through a layer filled with the ion-exchange resin whereby impurity ion in the raw water is removed so as to make the raw water to b deionized water which flows out through a deionized water outlet line 118.
The impurity ions permeate the membranes 104, 106, the concentrated water in the concentrating compartment flows out through a concentrate outlet line 114. Electrode water is passed within electrode compartments through introducing lines 115, 116 and discharging lines 117, 118, respectively.
An electrodeionization apparatus in which a diluting compartment is provided with vertical partition ribs for dividing the diluting compartment into cells being long in the vertical direction is disclosed in JP4-72567B. According to this electrodeionization apparatus having the diluting compartment divided into long cells by ribs in which ion-exchange resins are filled respectively, the channelizing phenomenon where the flow of water from the inlet to the outlet of the diluting compartment is partially one-sided is prevented and the compression and the ion-exchange resins in the diluting compartment are prevented from being compressed or moved.
In the electrodeionization apparatus of JP4-72567B, the number of the cells is limited because the cells are formed by dividing the diluting compartment in the vertical direction. That is, a large number of cells can not be formed in the apparatus. Further, the flow of the water in a lateral direction is blocked by the ribs, so that the contact efficiency between the water and the ion-exchange resins is poor. In addition, the ion-exchange resins are compressed at lower portions of the cells so that the cells have a vacancy at upper portions thereof, whereby the rate of filling the ion-exchange resins tends to be poor.
FIG. 4 is a system diagram showing a conventional apparatus for producing purified water provided with the electrodeionization apparatus in which raw water such as the city water is treated in an activated carbon treating device 1, a reverse osmosis membrane treating device 2, and an electrodeionization apparatus 3.
To remove weakly-ionized species electrolytes including carbon dioxide gas (CO2), silica, boron and the like in an electrodeionization apparatus, it is required to ionize these species and form ions as follows in diluting compartments: 
Even the conventional electrodeionization apparatus can completely remove weakly-ionized species having low dissociation constant (pKa) such as CO2 by increasing the applied voltage to dissociate water. However, the conventional electrodeionization apparatus scarcely removes weakly-ionized species having high dissociation constants such as silica and boron on the order of 60 to 90% even when the applied voltage is increased.
In order to solve above problems, the following have been proposed.
I. To fill diluting compartments with multi-layered ion exchangers composed of an anion exchange layer and a cation exchange layer so as to make the water alkaline temporarily in the anion exchange layer (as disclosed in JP-H471624A).
II. To adjust pH of feed water in a range of 9.5 to 11.5 to be fed into the electrodeionization apparatus (as disclosed is U.S. Pat. No. 4,298,442).
III. To provide the conventional electrodeionization apparatuses at two or more stages. To provide RO apparatuses at two or more stages to remove silica before the electrodeionization apparatus.
In the above case I, the diluting compartments filled with multi-layered ion exchangers cannot lower the concentration of silica to less than 0.1 ppb as required in the fields of the semiconductor manufacturing and the like.
In the above case II, although the removal rate of silica is increased by 5 to 10%, it requires a device for adding agents including caustic soda to control pH and to provide a softening device to completely remove the hardness including Ca2+ and Mg2+ from the feed water, whereby increasing equipment costs.
In the aforementioned case III, an electrodeionization apparatus of non-regenerative mixed bed type is necessary after the electrodeionization apparatus because the water treated by the electrodeionization apparatus includes silica and boron of 0.5 to 1.0 ppb or more.
Generally, when the electrodeionization apparatus is applied with the electrical current exceeding the critical current density to deionize, OHxe2x88x92 and H+ are formed by water dissociation as described above to carry the electric charge. H+ ion has mobility of 349.7 cm2xcexa9xe2x88x921eqxe2x88x921, which is very large in comparison with that of the other ions (30 to 70 cm2xcexa9xe2x88x921eqxe2x88x921, ref; Manual of Chemistry published by Japanese Chemical Society). Therefore, particularly when the diluting compartment has a large thickness W, the difference of the mobilities between H+ and OHxe2x88x92 is increased so that H+ tends to be quickly discharged to the concentrating compartments and OHxe2x88x92 tends to remain in the diluting compartment. Furthermore, Na+ and K+ also tend to remain in the diluting compartments because these are monovalent and H+ ion carries the electrons, while the multi-valent cations and anions including Ca2+, Mg2+ are discharged to the concentrating compartments with relative ease. As the result, the product water tends to include monovalent alkali such as NaOH and KOH so that the product water (deionized water) becomes alkaline.
Conversely, the concentrated water becomes acidic by the same reason.
It is an object of the present invention to provide an electrodeionization apparatus which overcomes the aforementioned problems so that the electrodeionization apparatus is free of scale and is exceedingly improved in the removal rate of the weakly-ionized species including silica, boron and the like without adding agents such as the caustic soda. Further, it is also an object to provide an apparatus for producing purified water employing the electrodeionization apparatus.
The electrodeionization apparatus of the present invention has an anode, a cathode, concentrating compartments, and diluting compartments which are formed by arranging a plurality of anion exchange membranes and cation exchange membranes between the anode and the cathode, ion exchangers filled in the diluting compartments. The electrodeionization apparatus can produce the product water having a pH higher than a pH of the feed water by 1.0 or more when the feed water having pH of equal to or less than 8.5 is treated without adding alkaline agent.
The electrodeionization apparatus of the present invention efficiently removes the weakly-ionized species including silica, boron from the feed water.
The apparatus for producing purified water of the present invention is provided with plural stages of the electrodeionization apparatuses through which the feed water flows in order. The foremost electrodeionization apparatus is the electrodeionization apparatus of present invention.
The foremost electrodeionization apparatus removes a part of carbon dioxide gas, silica, boron and the hardness from the feed water. The water treated in the foremost electrodeionization apparatus has the same conductivity as the feed water and higher pH than the feed water.
The water treated by the foremost apparatus is then treated by conventional electrodeionization apparatus to remove the residual silica, boron and the other ions.
When the diluting compartment has a thickness exceeding the range of 1.26 to 6.35 mm as disclosed in JP H4-72567B and is filled with either the anion exchanger alone or the anion exchanger mixed with the cation exchanger, the electrodeionization apparatus behaves specifically. That is, when the water including a small amount of alkaline metal ion or alkaline earth metal ion which are taken out of a reverse osmosis apparatus (RO apparatus) flows through the electrodeionization apparatus, the carbon dioxide gas (CO2) and the anion in the water are removed, besides about 90% of silica and boron are removed.
While the hardness including Ca2+ and Mg2+ is also removed, the monovalent cations including Na+ and K+ have difficulty in being removed and the alkalis such as NaOH and KOH which have high molar conductivity leak into the treated water so that the treated water tends to be increased in pH and slightly in conductivity.
The cause of the aforementioned behavior of the electrodeionization apparatus has not been clear in detail, but supposed as follows. That is, when the electrodeionization apparatus is applied with the electrical current in excess of critical current density to deionize, OHxe2x88x92 and H+ are formed by water dissociation as described above to carry the electric charge. H+ ion has mobility of 349.7 cm2xcexa9xe2x88x921eqxe2x88x921, which is very large in comparison with that of the other ions (30 to 70 cm2xcexa9xe2x88x921eqxe2x88x921, ref; Manual of Chemistry published by Japanese Chemical Society). Therefore, particularly as the thickness W of the diluting compartment increases, the difference of the mobilities between H+ and OHxe2x88x92 increases so that H+ tends to be unilaterally discharged to the concentrating compartments and OHxe2x88x92 tends to remain in the diluting compartment. Furthermore, Na+ and K+ also tend to remain in the diluting compartments because H+ ion carries the electrical charge, while the multi-valent cations and anions including Ca2+, Mg2+ are discharged to the concentrating compartments with relatively ease. As a result, the product water tends to include monovalent alkali such as NaOH and KOH so that the product water is increased in pH.
By the same reason, the concentrated water becomes acidic because of the tendency of OHxe2x88x92 to remain in the diluting compartment and the tendency of H+ to be discharged from the concentrating compartment. Therefore, the electrodeionization apparatus is free of scale even though ions such as Ca2+ and Mg2+ are concentrated at high concentration.
The electrodeionization apparatus of the present invention may be provided with a cation exchange membrane between the anode and the anion exchange membrane of the diluting compartments nearest to the anode, so that the concentrating compartments is formed between the cation exchange membrane and the diluting compartment nearest to the anode, and the anodic compartment is formed between the cation exchange membrane and the anode.
In the electrodeionization apparatus, the cation concentration in the cathodic compartment is high and the electric resistance between the electrodes is low, whereby the voltage applied to cells is decreased. To prevent formation of scale in the cathodic compartments, the electrodic water fed into the cathodic compartment is decreased in pH (or, the electrodic water is made acidic). For this purpose, the thickness of the diluting compartment is increased to decrease pH of the water which flows out from the concentrating compartments and then flows into the cathodic compartment as the electrode water.
The electrodeionization apparatus of the present invention is useful for a foremost electrodeionization apparatus of the apparatus for producing purified water in which two or more electrodeionization apparatuses are connected in series so as to make treatment of the feed water at plural stages. When feed water having conductivity of 10 xcexcS/cm and including silica 200 ppb and boron of 20 ppb flows through the first electrodeionization apparatus of the present invention and then flows through the second conventional electrodeionization apparatus, the water flowing out from the second electrodeionization apparatus has electrical resitivity of equal to or more than 18 Mxcexa9xc2x7cm and includes silica and boron of equal to or less than 0.1 ppb as like as the theoretical pure water. Since the preceding first electrodeionization apparatus removes the hardness such as Ca2+ and Mg2+, the succeeding second electrodeionization apparatus is free of scale and has the water recovery of equal to or more than 95%. The concentrating compartment of the preceding first electrodeionization apparatus produces the acidic water and the concentrating compartment of the succeeding electrodeionization apparatus produces the alkaline water. These acidic water and alkaline water may be mixed together and be fed back to the preceding RO apparatus.
The electrodeionization apparatus of the present invention may be provided with thick cells having a thickness of equal to or more than 7 mm and thin cells having a thickness of less than 7 mm as shown in FIG. 7 and FIG. 8 so that the water flows through from the thick diluting compartment to the thin cell in series.